Protheses or prothesis systems are being used more and more in the field of surgery. For example, typical applications concern the use in orthopedics of thighbone and hip protheses in the case of a fracture of the neck of the thighbone, arthritis of the hip, protheses for reparatory facial plastics surgery, orthopedics and other applications.
Like any foreign body introduced into the human or animal organism, the prothesis system needs to get integrated by the organism, and must also fulfill the functions of the deficient or replaced organ, in a manner which is fast, reliable, and durable.
So-called bio-compatible materials that are currently used for producing such prothesis systems for implant purposes are ceramics, Co--Cr--Mo alloys, and, currently, above all, titanium and alloys thereof. Thanks to the use of such bio-compatible materials, the bone prothesis system is only very slightly subject to the phenomenon of reject. Nevertheless, specific problems have occurred. Effectively, since these bone prothesis systems are implanted into a bone; for example, when a thigh bone prothesis is implanted into the intramedullary passage, it is necessary, in order for the prothesis system to be able to fulfill its functions effectively, rapidly, reliably, and durably, that fast, reliable, and durable growth of the bone take place at the interface around this prothesis system followed by subsequent bone bonding thereof. Thus, surface coatings for prothesis systems have been proposed, the function of which is to promote said growth or bone response at the bone/prothesis interface in order to lead to osseous accretion and to an osteo-integration, this being required to take place speedily, reliably, and durably.
Numerous surface coatings have been proposed, and those which have been adopted to date are based on calcium phosphate. Effectively, bone contains a major proportion of calcium phosphate and it would hence be natural to employ this for surface coatings as its nature and chemical structures are similar to those of bone. The various calcium phosphates are hydroxyapatite (HA), tricalcium phosphate (TCP) in its .alpha. and .beta. form (.alpha.-TCP and .beta.-TCP respectively) and several other calcium phosphates. These calcium phosphates differ from each other by their stoechiometry and their crystallographic properties. The calcium phosphate which currently comes closest to the osseous type is hydroxyapatite. The latter compound is now widely employed as a coating material for prothesis systems applied in thicknesses varying from several tens to several hundreds of .mu.m. Compounds such as TCP and mixtures thereof with HA are also employed along with other phosphates of calcium.
Unfortunately, as bone growth is an extremely complex phenomenon, present day phospho-calcium coatings for protheses do not currently provide complete satisfaction. Bone growth can be divided into two major reactions: a chemical reaction and a histological reaction.
The histological reaction firstly comprises a "cleaning up" stage carried out with the aid of the macrophages which clean up the surface of the implant by phagocytosis.
Following this, the histological reaction continues through colonization of the surface coating by cells, such as osteoblasts, fibroblasts, the previously mentioned macrophages etc., and by extra-cellular liquid. Colonization of the coating surfaces takes place. The extra-cellular components contain, among other things, proteinaceous fibers, such as collagen fibers. When the matrix settles down and is principally made up of collagen fibers, it is then highly differentiated and constitutes an osteoid.
The chemical reaction consists firstly of an extra-cellular dissolution. The solution comprised in the surface coating becomes enriched in calcium and phosphorous ions, released by the dissolving out of certain crystals. Following this, the released and dissolved ions precipitate. Such precipitation takes place in the extra-cellular fluid, this being a medium rich in proteins onto which the ions become fixed. The thus formed crystals take the form of needle-shaped crystals of biological apatites identical to those of bone. When the proteinaceous matrix is an osteoid, ion precipitation leads to osteocoalescence between the host tissue and the surface of the material of the prothesis. Osteo-genesis at the surface of the thus obtained surface coating is characterized by true bone affixed to the surface of the bioactive material constituting the prothesis.
Various coatings have been proposed and the one that is most frequently used is HA. This coating, however, suffers from the disadvantage of only being reabsorbable very slowly and of only being slightly bioactive. Certain authors have described HA as being a nonresorbable material, meaning that breakdown through the dissolving/precipitation process is very slow and, moreover, HA does not encourage mineralization on the collagen matrix, which is the ultimate aim being sought. Formation and accretion of true bone could be delayed. Coatings that are more bio-reactive than HA, in other words, that are semi-resorbable have been proposed. Among these, TCP is the most bio-reactive. Unfortunately, bio-degradation of TCP sometimes takes place too quickly, with resorption being too high. The osteoid has not yet formed when dissolving/precipitation takes place and hence a bone is not able to form. It seems to be necessary to have a mixed product available which associates the stability of Ha and the high bio-reactivity of TCP, like in applications where a two-phase macroporous material is used for bone filling.
Thus, two-phase HA/TCP or BCP (biphase calcium phosphate) systems have been proposed. BCPs in a 60/40 ratio correspond to the objectives of equilibrium between histological and chemical phenomena. Nevertheless, BCP still suffers from two disadvantages. Since the coating application technique makes use of a plasma torch or electrophoresis associated with a sintering operation, the crystalline form of the TCP can vary between the .beta. and .alpha. form, and as .alpha.-TCP is more soluble than the .beta. form, the properties of the BCP vary as a consequence. BCP obtained by plasma is hence highly bioactive. In the case of secondary complications, such as loosening of the implant in the receiving bone or septic complications, such complications could lead to the surface of the biocompatible material constituting the prothesis rapidly becoming bare. In this case, short-term fibrous encapsulation takes place and osteo-integration would become impossible.
Thus, there is no currently used coating that allows the above stated aims to be achieved effectively, such objectives requiting accretion and anchoring to the bone to take place rapidly, reliably, and durably.